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Abstract

Objective

To measure Mycoplasma pneumoniae pneumonia (MPP)-associated myocardial damage in different age groups of children with pneumonia.

Methods

Children aged 0–14 years with pneumonia and myocardial damage (serum creatine kinase isoenzyme-MB [CK-MB] concentration >25 U/l) were enrolled in the study. The children were classified as Mycoplasma pneumoniae immunoglobulin M positive (M. pneumoniae IgM+) or negative (M. pneumoniae IgM−) based on a serological test. Children were stratified into four age groups in order to analyse age-specific MPP-associated myocardial damage.

Results

The incidence of fever was significantly higher in children who were M. pneumoniae IgM+ compared with M. pneumoniae IgM− children. The median serum CK-MB concentration was significantly higher in children who were M. pneumoniae IgM+ compared with those who were M. pneumoniae IgM−. Children who were M. pneumoniae IgM+ in the 13–36 months and 72 months–14 years age groups had significantly higher median serum CK-MB concentrations than those who were M. pneumoniae IgM− in the same age group.

Conclusions

M. pneumoniae infection was associated with greater myocardial damage in children aged 13–36 months and 72 months–14 years. This suggests age-specific immune responses to M. pneumoniae.

Keywords

infection myocardial injury age immune system

Introduction

Mycoplasmas are the smallest self-replicating organisms capable of a cell-free existence, based on cellular dimensions and genome size.¹ Mycoplasma pneumoniae causes primary atypical pneumonia, tracheobronchitis, pharyngitis and asthma in humans.

Extrapulmonary complications occur at variable time periods following the onset of M. pneumoniae infections, even in asymptomatic infections, in up to 25% of people infected with M. pneumoniae.² Autoimmune reactions may be responsible for many of the extrapulmonary complications of M. pneumoniae infection.³ It is important to recognize that extrapulmonary complications can occur before, during or after pulmonary manifestations, or can occur in the complete absence of any respiratory symptoms.⁴

Cardiac complications associated with M. pneumoniae pneumonia (MPP) are relatively uncommon (1.0–8.5% of those with serological evidence of infection), and occur somewhat more commonly in adults than in children.^2,5,6 However, another study reported that almost half of patients had symptoms or signs of heart abnormalities at a mean of 16 months following M. pneumoniae infection.⁶ Although MPP-associated myocardial injury is well documented, there is a lack of research on the varying degrees of age-specific myocardial damage.

The aim of this present study was to measure the level of MPP-associated myocardial damage in different age groups of children with MPP compared with children without MPP.

Patients and methods

Patient population

This prospective study included all consecutive children aged 0–14 years old who were hospitalized with a diagnosis of pneumonia and who also had myocardial damage based on their serum creatine kinase isoenzyme-MB (CK-MB) concentration (>25 U/l) in the Department of Paediatrics, Tenth People’s Hospital, Tongji University, Shanghai, China between January 2011 and March 2012. The children were subsequently classified as M. pneumoniae immunoglobulin M positive (M. pneumoniae IgM+) or negative (M. pneumoniae IgM−).⁷ In order to analyse age-specific MPP-associated myocardial damage, patients were stratified into four age groups: infants (0–12 months); early childhood (13–36 months); preschool (37 months–71 months); school age (72 months–14 years old).

Children with tuberculosis, any prior heart disease or tracheal foreign bodies, or those who had received hormone therapy, nutrition-based cardiac treatment before hospitalization or intravenous immunoglobulin treatment within the previous 6 months were excluded from the study.

The study protocol was approved by the Clinical Ethics Committee, Tenth People’s Hospital, Tongji University (reference no. 2013-035). Written informed consent was obtained from the parents or legal guardians of the children who participated in this study and all relevant ethical guidelines were followed.

Laboratory analyses

Peripheral venous blood samples were collected after an 8-h fast in children >1 year of age. In children ≤1 year of age, blood samples were taken without a period of fasting. The blood was allowed to clot at 20 ℃ and centrifuged at 3000 g at 4 ℃ for 10 min in an Anke LXJ-ΙΙB centrifuge (Anting Scientific Instrument Factory, Shanghai, China). After the serum was separated, the presence of M. pneumoniae IgM was determined using a M. pneumoniae-specific immunogold filtration assay according to the manufacturer’s instructions (Kanghua Biotech, Weifang, China). The serum levels of CK-MB were measured using a Roche Modular Analytics DPP analyser (Roche Diagnostics, Mannheim, Germany).

Statistical analyses

All statistical analyses were performed using the SPSS® statistical package, version 17.0 (SPSS Inc., Chicago, IL, USA) for Windows®. The nonparametric Wilcoxon signed–rank test was used for comparisons of continuous variables and χ²-test was used for categorical data. A P-value <0.05 was considered statistically significant. As the data were not normally distributed and had a large standard deviation, the median and the interquartile range (25th, 75th percentiles) were calculated.

Results

This prospective study included 143 children aged 0–14 years who were hospitalized with pneumonia and myocardial damage based on serum CK-MB concentrations and who underwent further examinations for M. pneumoniae IgM positivity. In terms of M. pneumoniae IgM positivity, 70 children were classified as M. pneumoniae IgM+ and 73 were M. pneumoniae IgM−. In this present study, the youngest patient with M. pneumoniae-associated myocardial damage was 6 months old. The children were stratified into four age groups: infants (0–12 months; n = 29); early childhood (13–36 months; n = 53); preschool (37–71 months; n = 36); school age (72 months–14 years old; n = 25). Table 1 presents the clinical and demographic characteristics of the children categorized according to their M. pneumoniae IgM immunological status. There were no significant differences in the age, sex distribution or the season of diagnosis in the children who were M. pneumoniae IgM+ compared with those who were M. pneumoniae IgM−. A significantly higher proportion of children who were M. pneumoniae IgM+ had the symptom of fever compared with those who were M. pneumoniae IgM− (P = 0.002).

Table 1.

Demographic and clinical characteristics of 143 children who were hospitalized with pneumonia and categorized according to whether they were Mycoplasma pneumoniae immunoglobulin M positive (M. pneumoniae IgM+) or negative (M. pneumoniae IgM−).

Characteristic	Immunological status of children with pneumonia		Statistical significance^a
Characteristic	M. pneumoniae IgM+ n = 70	M. pneumoniae IgM−n = 73	Statistical significance^a
Age, months	45.9 ± 37.4	43.0 ± 37.7	NS
Sex
Male	37 (52.9)	45 (61.6)	NS
Female	33 (47.1)	28 (38.4)
Season of diagnosis
Spring	17 (24.3)	10 (13.7)	NS
Summer	9 (12.9)	8 (11.0)
Autumn	20 (28.6)	26 (35.6)
Winter	24 (34.3)	29 (39.7)
Fever present
Yes	63 (90.0)	50 (68.5)	P = 0.002
No	7 (10.0)	23 (31.5)

Data presented as mean ± SD or number of patients (%).

Wilcoxon signed–rank test was used for comparisons of continuous variables and χ²-test was used for categorical data.

NS, not statistically significant (P ≥ 0.05).

The median serum CK-MB concentration, a marker of myocardial damage, was significantly higher in children who were M. pneumoniae IgM+ compared with those who were M. pneumoniae IgM− (P = 0.02) (Table 2). Children who were M. pneumoniae IgM+ in the 13–36 month and 72 months–14 years age groups had significantly higher median serum CK-MB concentrations than those who were M. pneumoniae IgM− in the same age groups (P < 0.05 for both comparisons) (Table 3).

Table 2.

Serum creatine kinase isoenzyme-MB (CK-MB) concentrations in 143 children who were hospitalized with pneumonia and categorized according to whether they were Mycoplasma pneumoniae immunoglobulin M positive (M. pneumoniae IgM+) or negative (M. pneumoniae IgM−).

	Immunological status of children with pneumonia		Statistical significance^a
	M. pneumoniae IgM+ n = 70	M. pneumoniae IgM− n = 73	Statistical significance^a
Serum CK-MB, U/l	32.5 (28.0, 43.5)	30.0 (28.0, 35.0)	Z = 2.225; P = 0.02

Data presented as median and interquartile range (25th, 75th percentiles).

Wilcoxon signed–rank test.

Table 3.

Serum creatine kinase isoenzyme-MB concentrations (U/l) in different age groups of children (n = 143) who were hospitalized with pneumonia and categorized according to whether they were Mycoplasma pneumoniae immunoglobulin M positive (M. pneumoniae IgM+) or negative (M. pneumoniae IgM−).

Age group	Immunological status of children with pneumonia		Statistical significance^a
Age group	M. pneumoniae IgM+ n = 70	M. pneumoniae IgM− n = 73	Statistical significance^a
0–12 months	31.0 (29.0, 34.5)	34.0 (30.0, 37.0)	Z = 0.890
NS
13–36 months	37.0 (27.8, 45.0)	30.0 (28.0, 34.0)	Z = 2.226
P = 0.02
37–71 months	30.0 (27.3, 36.8)	28.0 (26.0, 36.3)	Z = 0.883
NS
72 months–14 years	41.0 (28.5, 49.5)	30.0 (27.5, 32.5)	Z = 2.047
P = 0.04

Data presented as median and interquartile range (25th, 75th percentiles).

Wilcoxon signed–rank test.

NS, not statistically significant (P ≥ 0.05).

Discussion

M. pneumoniae does not appear to present with a different clinical course based on the sex of the patient, but there has been considerable debate about whether or not there are age-specific behavioural characteristics associated with this infection.⁸ The incidence of M. pneumoniae infection is greatest among school-aged children of 5−15 years of age, with a decline after adolescence.^9–11 However, recent clinical research has reported a greater prevalence in younger children.⁸ M. pneumoniae was identified using polymerase chain reaction analysis of nasopharyngeal aspirates of newborn infants who developed pneumonia shortly after birth, which suggested that M. pneumoniae can be added to the long list of pathogens known to cause congenital pneumonia.¹² According to a case report, maternal serum tested negative for both maternal IgM and IgG antibodies to M. pneumoniae, while the serum of a 3-week old baby had high IgM titres to M. pneumoniae.¹³ It is possible that the baby’s susceptibility to M. pneumoniae was due to the lack of protective maternal IgG antibodies against M. pneumoniae.

Cardiac complications associated with M. pneumoniae are relatively uncommon, with a prevalence rate of 1.0–8.5% and a slightly higher prevalence rate in adults than children.^5,6 According to one study,⁶ almost half of patients with M. pneumoniae had symptoms or signs of heart abnormalities at a mean of 16 months after infection. Constrictive pericarditis secondary to infection with M. pneumoniae has also been reported.¹⁴ In this present study, the youngest patient with M. pneumoniae-associated myocardial damage was only 6 months old. Thrombosis formation and transient production of antiphospholipid antibodies may be responsible for M. pneumoniae-associated myocardial pathology.¹⁵ It has been suggested that Mycoplasma species are potential causes of myocardial disease because of their ability to cause chronic disease and to participate in the formation of atheromatous plaques.¹⁶

M. pneumoniae was first isolated in a sputum sample from a patient with primary atypical pneumonia by Eaton in 1944, and thereafter it was known as Eaton’s agent.¹⁷ M. pneumoniae infection is common in children and adolescents.^17,18 Extrapulmonary complications, which can involve all of the main organ systems, can occur as a result of the direct invasion of the bacteria and/or the induction of an autoimmune response.^2,16 M. pneumoniae has antigens that share some similarity with antigens present on human organs such as the heart, lung, liver, brain and kidney.^2,19,20 Consequently, the body produces antibodies that organize and form immune complexes that activate the complement system.^19,20 During M. pneumoniae infections, cytokines may also be released from various cell types.²¹

The serological test for IgM antibodies to M. pneumoniae has been reported to be the single most valuable tool for the diagnosis of M. pneumoniae infection in children of any age,²² so this test was used for diagnosis in the present study. The normal range of serum concentrations of CK-MB in children and the levels that indicate myocardial damage remain unknown, so the present study used the diagnostic criteria for adults (i.e. CK-MB > 25 U/l) in order to diagnose myocardial damage.^23,24 The present study found a significantly higher serum concentration of CK-MB in children who were M. pneumoniae IgM+compared with those who were M. pneumoniae IgM−, suggesting that there was more myocardial damage in the children with MPP. When the children were stratified according to age, children who were M. pneumoniae IgM+ in the 13–36 months and 72 months–14 years age groups had significantly higher serum CK-MB concentrations than those who were M. pneumoniae IgM− in the same age group, suggesting that the myocardial damage was greater in these two age groups. The clinical manifestations of MPP appear to differ for early childhood and school-age children.^8,12,25 In infants, the main symptoms are cough, asthma and pulmonary signs, and chest radiographs demonstrate patchy and fan-shaped infiltration.^8,26,27 However, school-age children have high rates of fever and dry cough, with main stem pulmonary changes evident in chest radiographs.^8,26,27 A possible explanation for why different age groups have different clinical manifestations associated with M. pneumoniae infection is that there are different age-specific immune responses to M. pneumoniae.^8,26,27

The blood cells of infants are primarily lymphocytes, which account for nearly 60% of their blood cells, but this ratio changes at 4–71 months of age when the numbers of peripheral blood lymphocytes and neutrophils become similar.²⁸ Although neutrophils predominate in children aged >6 years, the lymphocytes are more mature.²⁸ There are also age-specific variations in immunoglobulin production.²⁹ Infants with MPP have higher levels of IgG, IgM and circulating immune complexes (CIC), and lower serum total complement activity and complement 3 levels compared with children without MPP.²⁹ These previous findings suggest that immune complex-mediated immune reactions may cause MPP-related pathology. MPP-associated antigen may stimulate the body to produce specific antibodies that form immune complexes that promote immune cell responses, such as increased CIC levels.^29,30 This inflammatory damage causes immune-mediated organ-specific symptoms and dysfunction that result in multiple organ damage.^20,21 Young children with MPP have T cell functional disorders,³¹ and infants with MPP have lower levels of cluster of differentiation (CD)3⁺ and CD4⁺ T cells, and lower CD4⁺/CD8⁺ ratios.³² School-age children have well-developed humoral and cellular immune systems that can facilitate immune system damage. An imbalance in lymphocyte-mediated T cell immunity and release of large numbers of cytokines such as interleukin (IL)-4, IL-5 and tumour necrosis factor occurs in patients with M. pneumoniae infections.^20,33,34 This may contribute to multiorgan damage in school-age children.

This present study had a number of limitations. First, it had a limited number of patients, especially in the infant and school-age groups. In addition, the study only included children from Shanghai, China. Although extrapulmonary complications associated with M. pneumoniae infections are reported elsewhere in the world, these current findings cannot necessarily be extrapolated to other countries without further research, in particular the variations in myocardial damage that were observed based on age.

In conclusion, this present study demonstrated that M. pneumoniae infections cause more serious myocardial damage in children aged 13–36 months and 72 months–14 years as detected using serum CK-MB concentrations. This suggests a need for increased attention to age-specific differences in immune system responses to M. pneumoniae infection. Future studies need to have larger numbers of cases in order to clarify the aetiology of these preliminary observations on the cardiac complications of M. pneumoniae infections. These current findings also suggest that paediatricians should pay more attention to the extrapulmonary complications associated with M. pneumoniae infections.

Footnotes

Declaration of conflicting interest

All authors declare that there are no conflicts of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Authors' contributions

Li Gu had responsibility for the study concept and design, and writing the manuscript. Cheng-Mei Li participated in the preliminary data collection and data analysis, and wrote the manuscript. Shao-Jun Yin supervised the design and execution of the study. Rong Yang, Xiao-Zhi Guo, Yu-Xuan Fu and Dan Cheng participated in patient care, data collection and data analysis. Yuan Xie carried out the serological immune test. All authors read and approved the final manuscript.

References

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Age-specific Mycoplasma pneumoniae pneumonia-associated myocardial damage in children

Abstract

Objective

Methods

Results

Conclusions

Keywords

Introduction

Patients and methods

Patient population

Laboratory analyses

Statistical analyses

Results

Discussion

Footnotes

Declaration of conflicting interest

Funding

Authors' contributions

References